Consistent and reliable etch processing is critical to the fabrication of integrated circuits (ICs). Etch processing is used to remove portions of layers of material to perform tasks such as forming contacts and vias, planarizing a layer of material, forming trench isolation, etching and defining interconnection layers, determine the size and shape of transistors, self-align portions of multiple layers of material, forming sidewall spacers, and is in general extensively used in IC processing. Integrated circuits are usually exposed to tens or hundreds of etch processing steps during their manufacturing cycle.
Most etch processing involves masking a layer of material with a lithographically-patterned masking layer, such as photoresist. The masking layer defines exposed portions of the layer of material and unexposed portions of the layer of material. The exposed portions of the layer of material are exposed to a chemical and/or mechanical etch process which may be a plasma etch, an isotropic etch, an anisotropic etch, a reactive ion etch (RIE), or like etch processes. The quality of an etch process is very dependent upon consistency, selectivity, and repeatability. For example, an etch process should etch across a wafer and an IC die in a uniform manner. The etch process should be able to etch one type of material (i.e. oxide) without significantly damaging or removing other materials (i.e. metals, silicon, polysilicon, nitride etc.). Furthermore, the etch process (i.e. etch rate, sidewall profile, selectivities, etc.) should not vary with time.
In order to achieve repeatability and consistency, an etch process may be performed in a timed manner (i.e. a timed etch). In a timed etch, an engineer assumes that the layer of material which requires etching is always consistently formed. For example, the layer of material is consistent in thickness, consistent in chemical composition, consistent in impurity concentration, consistent in topography, and the like. The engineer then calculates, given a fixed condition of the etch system (i.e. fixed temperature, fixed pressure, fixed gas flows, etc.), how much time it will take to properly etch the layer of material. The layer of material is then etched for the calculated time interval and hopefully the layer of material is properly etched. In many cases, after the timed etch is completed, a second timed etch is performed which is known as a "soft etch" or an "over etch." The over etch is a second etch process which continues beyond the timed etch to ensure proper etching of the layer of material. The over etch is usually an etch that is less aggressive (i.e. slower etch rate, better selectivities, etc.).
No process is completely consistent. Therefore, the thickness, chemical composition, impurity concentration, topography, and the like, of the layer of material will vary from die to die, wafer to wafer, machine to machine, and will vary across time as conditions change (i.e. H.sub.2 O outgassing, polymerization of the etch chamber, changes in the flow of gases, etc.). Variations in etch processing, such as pressure variation, temperature variations, etc. are also possible. When variations in the layer of material and/or variations in the etch process occur, the timed etch may not etch properly, open circuits may occur where contacts were to be formed, excessive damage may be caused to underlying other exposed layers of material, photoresist loss may increase, short circuits may occur due to over-etching, and planarization processing may under-etch or over-etch the layer of material. Problems associated with microloading and macroloading are increased, and other known problems may also result. In general, timed etches are not adequately controlled to overcome inherent variations in integrated circuit processing.
Other etch processes include a resist etch-back technique which is used to form a planarized or smoothed layer of material. In general, a nonplanar layer of material is formed overlying a substrate. A planar photoresist layer is formed overlying the nonplanar layer of material. The photoresist layer and the layer of material are exposed to a timed-etch chemistry which etches the layer of material and the photoresist at the same etch rate, for example 500 Angstroms per minute. The planar surface profile of the photoresist is transferred/etched into the layer of material resulting in a planar layer of material.
As discussed above, variation in the layer of material, photoresist, and processing conditions, may result in the etch rate of the layer of material and the etch rate of the photoresist being unequal. The layer of material may then be over-etched, under-etched, and/or not completely planarized. Therefore, conventional resist etch-back techniques are not as reliable and consistent as desired.
To ensure consistent etch processing, endpoint detection may be used. Endpoint detection is an alternative to timed-etch processing. Endpoint detection methods monitor the optical spectrum of an etch chemistry environment. When various signals from the optical spectrum approach a certain rate of change or a certain absolute level, the etch is completed. Although endpointing results in a more reliable completion to an etch process, the endpoint technique does nothing to control the etch chemistry during the etch process. Therefore, etch rates, planarization, sidewall profiles, and the like are not always consistent.
In some cases, a process more advanced than endpointing: may be used to track etch progress. In some cases, the entire optical emission versus time of an entire etch process (i.e. from start to endpoint) is stored by a computer. After the etch is complete, the computer or a human user may compare the optical emission versus time to an optimal optical emission plot to determine if the etch was performed in an optimal manner. If the optical emission versus time is not optimal then an engineer is informed that the etch process is out of specification. Unfortunately, once the optical emission is compared in the computer, the etch has already been completed and the damage is already done. Subsequent process modifications will not repair the damage already done.
Another method which is used to improve etch process performance is a method known as a two-step etch. In general, a first etch chemistry or a first plurality of etch environment conditions is used to etch a layer of material for a fixed amount of time or until an endpoint is achieved. After the first etch chemistry completes its etching, a second etch chemistry or a second plurality of etch environment conditions is applied to the layer of material for a fixed time or until an endpoint is achieved. The two-step etch process is used to achieve improved selectivity but is not used to accurately control an etch process. In general, the two-step etch process is performing two back-to-back conventional etches and controlling and endpointing the etch environment in conventional manners (i.e. endpointing or timed etch).
In general, the methods described above are static solutions to a dynamic problem of varying equipment tolerances and specifications, varying process conditions, varying ambient environmental parameters, human error, gas flow variations, topographical variation, known loading effects, etc..